Electronic Theses and Dissertations (PhDs)
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Item Transfer reactions to populate the pygmy dipole resonance in 96Mo(University of the Witwatersrand, Johannesburg, 2023) Khumalo, Thuthukile Charmane; Pellegri, L.; Wiedeking, M.The presence of a low-lying dipole strength in neutron-rich nuclei has been established and its location in the vicinity of the neutron threshold (Sn) has implications in nucleosynthesis and specifically in neutron-capture reaction rate calculations. Additionally, a correlation of this low-lying dipole strength with neutron-skin thickness has been discussed. Since its observation, there has been a great deal of work in an attempt to understand its nature, both theoretically and experimentally. Some of the characteristics of this low-lying dipole strength include isospin mixing, which allows the use of different experimental probes to study it. In addition, compared to the IVGDR, the degree to which the low-lying dipole states are collective is under scrutiny and remains an open question of interest. This study was aimed at addressing the question of collectivity of these dipole states and one-nucleon transfer reactions were the chosen probes as they have been shown to be powerful in probing the single-particle property of nuclei. In particular the (p,d) and (d,p) reactions have been instrumental in such measurements. To allow the investigation from both neutron addition and removal, the 96Mo nucleus is particularly attractive as it can be populated via both mechanisms, with the availability of stable targets as a bonus. In addition, the (d,p) has been successfully used recently used for PDR related measurements on 120Sn and 208Pb with results alluding to a strong single-particle contribution, hence conducting the investigation on 96Mo provides access to a different mass region. 97Mo(p,d)96Mo and 95Mo(d,p)96Mo transfer reactions were performed in normal kinematics using the MAGNEX magnetic spectrometer at INFN-LNS. The 25 MeV/u proton beam and 5 MeV/u deuteron beam from the Tandem accelerator interacted with the 97Mo and 95Mo targets, respectively. The MAGNEX spectrometer was utilised to analyse the scattered particles based on their momentum prior to being detected at the focal-plane. Excitation energy spectra were obtained and angular distributions were computed for the bound states and the higher excitation energy region of interest (above Ex = 4 MeV). These were fitted, using the MDA with DWBA calculations considering different single-particle configurations from a simplistic shell model. Comparing spectra from the two reactions, same excitation energy regions were populated. The results from the MDA of the (p,d) data, show a strong single-particle component in the Ex region that was analysed, with one particular configuration that excites 1− states dominating. The QPM was used for the theoretical interpretation and below 6 MeV, the configuration ((2d5 2 )+1 N(1g9 2 )−1) that populates 2+ states dominates but in the experimental data, this configuration was found to be suppressed as the momentum matching conditions were optimized for l=1 momentum transfer. When considering the QPM predictions involving only the sp configurations of momentum transfer of l=1, 2 and 3, an agreement with the data was found. Extraction of reliable angular distributions from the (d,p) was not possible thus future (d,pγ) experiments are envisagedItem Optimization of gallium oxide (ga2o3) nanomaterials for gas sensing applications(University of the Witwatersrand, Johannesburg, 2024) Gatsi, Nyepudzai CharslineGas sensors are needed for monitoring different gases in indoor and outdoor environments, food quality assessment, and health diagnostics. Among materials studied for these applications, semiconducting metal oxides (SMOs) have generated a lot of interest due to their excellent sensitivity, simple circuit, and low cost. One-dimensional (1𝐷) 𝐺𝑎2𝑂3 nanomaterials are part of the promising candidates explored for the sensing of different gases due to their excellent electrical conductivity, high catalytic behavior, and chemical and thermal stability. This study reports the optimization of crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for use in the detection of various gases. A set of unmodified and noble metal modified 1𝐷 𝐺𝑎2𝑂3 nanomaterials were synthesized by microwave-assisted hydrothermal method followed by heat-treatment at different temperatures and their gas sensing performances were systematically studied. The samples were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman analysis, scanning electron microscope (SEM), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET), photoluminescence (PL), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) methods. The effects of heat-treatment temperatures on phase transformations and gas sensing performances of various 𝐺𝑎2𝑂3 polymorphs were investigated. The 𝛼 − 𝐺𝑎2𝑂3, 𝛽 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures were synthesized and evaluated for gas sensing. The 𝛽 − 𝐺𝑎2𝑂3 sensing layers presented selective response coupled with fast response/recovery times towards carbon monoxide (𝐶𝑂) compared to the 𝛼 − 𝐺𝑎2𝑂3 and 𝛼/𝛽 − 𝐺𝑎2𝑂3 crystal structures. The observed variations in the gas sensing performances of these three crystal structures were attributed to controlled properties of different 𝐺𝑎2𝑂3 polymorphs. Furthermore, the 𝛽 − 𝐺𝑎2𝑂3 polymorph was prepared in the form of regular and hierarchical nanorod-based morphological features which demonstrated different gas sensing behaviors. The 𝛽 − 𝐺𝑎2𝑂3 regular nanorods showed better capabilities of detecting isopropanol than the nanobundle-like and nanodandelion-like features, and these differences were attributed to changes in textural, porosity, and compositional properties related to different morphologies. The effects of incorporating 𝐴𝑔 and 𝐴𝑢 noble metal nanocrystals on regular 𝛽 − 𝐺𝑎2𝑂3 nanorods surfaces on their gas sensing behaviour were also investigated. The results revealed that surface modification of 𝛽 − 𝐺𝑎2𝑂3 nanorods with 0.5 and 1.0 𝑚𝑜𝑙% 𝐴𝑔 and 𝐴𝑢 noble metals significantly lowered the sensor operating temperature compared to that of unmodified 𝛽 − 𝐺𝑎2𝑂3 nanorods towards the detection of ethylene. In addition, surface incorporation of 1.0 𝑚𝑜𝑙% 𝐴𝑔 dramatically increased the sensor sensitivity and selectivity and reduced the response/recovery times towards ethylene gas, and these positive changes were attributed to the electronic and chemical sensitization effects stimulated by the catalytic activity of 𝐴𝑔 nanocrystals incorporated on the surface of 𝛽 − 𝐺𝑎2𝑂3 nanorods. This study unambiguously optimized the crystal structure, morphology, and surface chemistry of 𝐺𝑎2𝑂3 nanostructures for the detection of carbon monoxide, ethylene and isopropanol gases. These sensors may potentially be used in real-time detection of carbon monoxide and isopropanol for indoor air quality monitoring to improve human health. In additional they have also demonstrated capabilities for the precise and economical detection of ethylene around plants and fruits, which could be beneficial to the horticultural and agricultural industriesItem Modification of boron nitride nanostructures induced by medium energy ion irradiation(University of the Witwatersrand, Johannesburg, 2023-08) Lisema, Lehlohonolo Innocent; Madhuku, Morgan; Derry, TrevorThis research focused on using Chemical Vapour Deposition (CVD) to synthesize boron nitride nanostructures, particularly nanotubes, and selectively introducing defects into them through ion implantation. Boron ion implantations were carried out at ambient temperature at 150 keV energy and fluences 1x1014 and 5x1014 ions/cm2. The synthesized samples were analyzed using scanning electron microscopy (SEM), Raman spectroscopy, and Grazing incidence X-ray diffraction (GIXRD). Ion implantation was found to introduce defects into the surface of the samples, resulting in increased stress levels and a higher local density that favoured more crystallized nanostructures. SEM images showed clear evidence of BN nanostructures and boron nitride nanotubes (BNNTs), with the latter appearing as long, thin structures with diameters ranging from ⁓30-90nm. After ion implantation, the Raman spectra of samples implanted with ion fluence 5×1014 ions/cm2 at 1000oC, show an amorphous h-BN peak, and a narrower, intense E2g vibrational mode of h-BN is observed around 1366 cm-1 for samples synthesized at 1100oC and 1200oC. Raman analysis did not show any E2g mode of vibration of h-BN for all samples at implanted with ion fluence 1×1014 ions/cm2. The samples synthesized at 900 ºC had no active 1366 cm-1 Raman peak present. Grazing incidence X-ray diffraction (GIXRD) spectra revealed a prominent peak between 54 and 56 ° 2θ, corresponding to the (004) h-BN reflection, which was used to determine the average a and c lattice parameters 0.249 ± 0.0002 nm and 0.662 ± 0.001 nm, respectively, yielding an interplanar distance of 0.166 ± 0.0001 nm representing the stacking direction of the BN layers. The majority of the samples had broad peaks, indicative of a nanocrystalline material. The only exception was the sample grown at 1200 °C, which was found to have a Scherrer crystallite size >100 nm. In contrast, the rest of the samples had an average size of 3.5 ± 0.3nm. The average crystalline domain size values confirmed that after ion implantation, the phonon lifetime would be longer due to a large domain size, indicating that the BN nanostructures were more crystallized. The fluence of 5x1014 ions/cm2 showed to be the optimal growth condition for BNNTs. Overall, BNNTs and BN nanostructures were effectively synthesized at 900°C, 1000°C, 1100°C, and 1200°C CVD temperatures, and insights into the influence of ion implantation on the composition as well as properties of BN nanostructures are presented. The most noteworthy finding of the experiment was the substantial increase in the size of the Raman derived crystallite domains in the 1100°C and 1200°C samples following ion implantation with boron ions at a fluence of 5x1014 ions/cm2.Item Search for new resonances in the four-lepton channel and implementation of the LED integrator panel for the PROMETEO system in the ATLAS Tile Calorimeter(University of the Witwatersrand, Johannesburg, 2024) Mtintsilana, Onesimo; Kumar, Mukesh; Mellado, BruceThe Large Hadron Collider (LHC) has transformed our understanding of fundamental particles and forces, notably with the seminal discovery of the Higgs boson in 2012, which completed the Standard Model (SM) of particle physics. Despite its success, the SM leaves numerous unanswered questions, motivating a quest for new physics. This thesis explores three main avenues: Firstly, it investigates the possibility of an extended Higgs sector or alternative SM extensions, focusing on heavy ZZ resonances that decay into four leptons. Using a dataset of 139 fb−1 from proton-proton collisions at the LHC, this study explores both gluon-gluon fusion and vector-boson fusion production mechanisms. Although no significant signal for a new resonance is observed, upper limits on the production cross section of spin-0 or spin-2 particles are established. These limits provide constraints on specific theoretical models, such as Type-I and Type-II two-Higgs doublet models for spin-0 resonances, and the Randall-Sundrum model for spin-2 resonances. Intriguingly, the combined results of ATLAS and CMS for Run 2 and Run 3 data in the final state of 4 leptons exhibit an excess around 250 GeV, reaching a significance of 2.4σ which is in the region of interest of the multi-lepton anomalies.. In the second part, the analysis extends to heavy boson decays resulting in a final state with four leptons, specifically focusing on the R boson or the A boson decays into a combination of the SM Higgs boson and another boson, denoted S, which further decays into dark-matter candidates. No evidence contradicting SM predictions is found, yielding stringent upper limits on the production cross-sections of these hypothesised bosons and their branching ratios at a 95% confidence level. Lastly, the thesis highlights advancements in Higgs boson studies and new particle discovery potential in the upcoming High-Luminosity LHC era starting in 2029, emphasising improvements to the ATLAS detector electronics, particularly the integration of a new LED Integrator Panel within the Prometeo portable readout module system, enabling precise calibration and monitoring of individual detector componentsItem The application of weakly supervised learning in the search for heavy resonances at the LHC(University of the Witwatersrand, Johannesburg, 2023-06) Choma, Nalamotse Joshua; Ruan, Xifeng; Mellado, BruceThe discovery of the Higgs boson at the Large Hadron Collider by the ATLAS and CMS experiments has made the search for new physics beyond the Standard Model a priority in the field of High Energy Particle Physics. New resonances have yet to be discovered using inclusive and model-dependent searches, which means they may be driven by subtle topologies. Rapid improvements in Machine Learning techniques have led to their increasing application in High Energy Particle physics. Unlike supervised learning, which is known to assume full knowledge of the underlying model, semi-supervised learning, in particular weakly supervised learning, allows the extraction of new information from data with partial knowledge. The goal of this study is to set up searches for heavy resonances at the electroweak scale with topological requirements performed in both inclusive and exclusive regions of phase-space tailored to a particular production mode. These resonances could be generated with different production mechanisms. In this work, we describe search procedures based on weakly supervised learning applied to mixed samples and used to search for resonances with little or no prior knowledge of the production mechanism. This approach has the advantage that sidebands or control regions can be used to effectively model backgrounds without relying on models. The effectiveness of this method is measured by the production of the Standard Model Higgs boson, which decays into a pair of photons in both inclusive and exclusive regions of phase-space at the LHC. Having confirmed the ability of the method to extract various Standard Model Higgs boson signal processes, the search for new phenomena in high mass final states will be set up at the LHC. Subsequently, the approach is used in the search for new resonances in the Zγ final state with Z → e +e − or Z → µ +µ −, using the Monte Carlo simulated signal samples for 139 fb−1 of integrated luminosity for Run 2 collected at the LHC. The weakly supervised learning approach is implemented and compared to the performance of the fully supervised approach, which is then used to calculate the production limit for Higgs-like particles for Zγ where the significance of the signal is maximal.